From the viewpoint of global environmental protection, improvement in the fuel economy of automobiles has recently been required to regulate the amount of CO2 emissions, thus requiring weight reductions due to a reduction in the thickness of members used. In addition, it is also required to improve safety by focusing on the crashworthiness of automobile bodies to ensure the safety of passengers at the time of a crash. Thus, both the weight reduction and strengthening of automobile bodies are being actively promoted. To simultaneously achieve the weight reduction and strengthening of automobile bodies, an increase in the strength of a material for members to the extent that stiffness is not impaired and a reduction in weight by reducing the thickness of sheets are said to be effective. Nowadays, high-strength steel sheets are positively used for automotive parts. The use of higher-strength steel sheets results in a more significant weight reduction effect. There is a trend toward the use of steel sheets having a tensile strength (TS) of 540 MPa or more for frame members for heavy vehicles, such as frames for trucks and construction machines.
Many automotive parts made from steel sheets are manufactured by press forming. Regarding the formability of high-strength steel sheets, dimensional accuracy is important in addition to prevention of cracking and wrinkling. In particular, the control of springback is an important problem. Nowadays, new automobiles are developed very efficiently by computer assisted engineering (CAE). So, it is not necessary to make many dies. Furthermore, the input of the characteristics of a steel sheet enables us to predict the amount of springback more accurately. However, in the case of large variations in the amount of springback, prediction accuracy by CAE is disadvantageously reduced. So, in particular, a high-strength steel sheet having only small variations in strength and excellent uniformity in strength is required.
As a method for reducing variations in strength in a coil, Japanese Unexamined Patent Application Publication No. 2004-197119 discloses a method in which a sheet bar composed of precipitation strengthened steel containing Cu, Ni, Cr, Mo, Nb, V, and Ti is subjected to hot finish rolling, air cooling for 1 second or more, and coiling at a temperature ranging from 450° C. to 750° C., so that variations in strength are within ±15 MPa in the longitudinal direction of the resulting coil. Japanese Unexamined Patent Application Publication No. 2002-322541 discloses a high-strength hot-rolled steel sheet with only small variations in strength and excellent uniformity in strength, the steel sheet being produced by the combined addition of Ti and Mo to form very fine precipitates uniformly dispersed therein.
The foregoing related art, however, has problems such as described below. In the method described in Japanese Unexamined Patent Application Publication No. 2004-197119, addition of Nb and Mo causes an increase in cost, which is economically disadvantageous.
In a steel sheet to which Ti, V, and Nb are added to increase the strength, if the temperature of the steel sheet is high after hot finish rolling, coarse precipitates are formed by strain-induced precipitation. So, disadvantageously, the additive elements need to be further added.
Although the steel sheet described in Japanese Unexamined Patent Application Publication No. 2002-322541 is a Ti-containing steel sheet, it is necessary to add Mo, which is expensive, thus causing an increase in cost.
Moreover, in any of the above-described publications, two-dimensional uniformity in strength in the in-plane directions including both of the width direction and the longitudinal direction of the coil is not taken into consideration. Disadvantageously, even if the coiling temperature is uniformly controlled, the variations in the in-plane strength of the coil are inevitably caused by different cooling histories for each position in the coiled coil.
In consideration of the above-described situation, it could be helpful to provide a high-strength hot-rolled steel sheet using an inexpensive, general-purpose Ti-containing steel sheet without using expensive additive elements, e.g., Ni, Nb, or Mo, the steel sheet having a tensile strength (TS) of 540 MPa or more, only small variations in strength, and excellent uniformity in strength in a hot-rolled coil.